1. Field of the Invention
This invention relates to a motor generating mechanical power without resorting to an electromagnetic force, and in particular to a motor which utilizes a circular motion excited in a vibrator by the combination of expansion and contraction vibrations in the axial direction to rotate a driven member coaxially fitted to the vibrator by frictional driving.
2. Related Background Art
Vibrators of this kind are disclosed, for example, in Japanese Patent Laid-Open Application No. 62-141980 and Japanese Patent Laid-Open Application No. 63-214381.
Such a vibrator, as shown in FIG. 12 of the accompanying drawings, comprises a vibration member 100 comprising a metallic round bar of cone-like shape the outer diameter of whose base portion gradually decreases toward the fore end portion, a circular ring-like metallic keep member 101 having an outer diameter equal to that of the large-diameter portion of the vibration member 100, and two electrostrictive element plates 102, 103 as electromechanical energy conversion elements formed into a circular ring-like shape and disposed between the vibration member 100 and the keep member 101, the keep member 101 being fixed to the vibration member 100 by a bolt 104, the electrostrictive element plates 102 and 103 being urged against each other and held thereby. The electrostrictive element plates 102 and 103 each have on one surface thereof two electrodes differing in the direction of polarization from each other and formed symmetrically divisionally and have a common electrode on the other surface thereof, and are disposed with a positional phase of 90.degree. each and with the divisional electrode side as the front side. Between the electrostrictive element plates 102 and 103, there are disposed electrode plates 105 and 106 which are in contact with the divisional electrodes of the rearward electrostrictive element plate 103 and the common electrode of the forward electrostrictive element plate 102, and the divisional electrodes of the forward electrostrictive element plate 102 are in contact with the vibration member 100 and the common electrode of the rearward electrostrictive element plate 103 is in contact with a common electrode plate 107.
By AC voltages equal in both amplitude and frequency being applied to the forward electrostrictive element plate 102 and the rearward electrostrictive element plate 103 with a phase difference in time therebetween, vibration comprising the combination of the vibration of the electrostrictive element plate 102 and the vibration of the electrostrictive element plate 103 is generated in the vibrator to thereby cause the fore end of the vibrator to effect circular motion.
FIG. 13 of the accompanying drawings shows a motor using such a vibrator as a drive source. The fore end of the vibrator is urged against the surface of a disk 108, and the disk 108 is frictionally driven by the circular motion of the fore end of the vibrator and a rotative force is output from a rotary shaft 109 fixed to the center of the disk 108.
Now, the motor utilizing such a vibrator utilizes the movement of the fore end portion of the vibrator, and according to my experiment, such motor has the difficulty that the rotational torque of the fore end portion of the vibrator is weak and a sufficient driving torque cannot be given to the disk which is a driven member.
The following two points have been considered to be the reasons therefore. The first point is that the fore end portion is a free end having no node at one side thereof and is low in rigidity and therefore sufficient vibration energy is not transmitted. The second point is that the free end which is the fore end is the loop of the vibration of rope skipping motion generating a circular motion (i.e., the vibration of a motion similar to the motion of the rope in rope skipping) and at the same time, the loop of the vibration in the axial direction. Therefore, the circular motion of the fore end in a plane perpendicular to the axis which is asserted by the aforementioned Japanese Patent Laid-Open Application Nos. 62-141980 and 63-214381 does not actually take place and there is presented a form of frictional driving which is not smooth and in which the vibrator contacts the moving member only once per period of the vibration.
Also, in the case of such a motor in which the vibrator effects the aforedescribed vibration of rope skipping motion, it is necessary to give sufficient consideration also to the supporting of the driven member so that the motor may not become bulky.